Monopropellant, as the name implies, is a single propellant ingredient which provides specific impulse (Isp) as it undergoes decomposition from combustion or decomposition by other means such as by catalytic decomposition. A monopropellant generally has a high freezing point and a low Isp. These properties therefore exclude the use of monopropellants in certain tactical situations.
An extensively used monopropellant such as hydrazine has been used as a rocket fuel. Hydrazine has a melting point of 2.degree. C. Hydrazine dissolves many inorganic substances and forms salts with inorganic acids. For example, hydrazine nitrate, N.sub.2 H.sub.4. HNO.sub.3, has been employed with hydrazine and water to yield a higher specific impulse fuel blend having a lower freezing point. Hydrazine is miscible with water, methyl, ethyl, propyl, and isobutyl alcohols. Hydrazine forms an azeotropic mixture with water with a boiling point at 760 mm of mercury of 120.3.degree. C.
Although adding water to hydrazine lowers the freezing point of the mixture, the addition of a nonenergetic material defeats the purpose of achieving a high specific impulse while lowering the freezing point. A tactical monopropellant should have a wide temperature range for use above and below the standard freezing point for water.
A preferred combination for a tactical monopropellant fuel blend is a monopropellant fuel blend whereby the freezing point is lowered by an additive that is multifunctional, i.e., the additive for making a monopropellant fuel blend lowers the freezing point and also raises the specific impulse.
Therefore, an object of this invention is to provide an additive to a monopropellant which forms a miscible monopropellant fuel blend having a lower freezing point and a higher specific impulse as compared with a standard monopropellant blend of hydrazine, hydrazine nitrate, and water.
A further object of this invention is to provide a monopropellant fuel blend wherein the ingredients are soluble or miscible in sufficient amounts to contribute to the specific impulse while lowering the freezing point of the monopropellant fuel blend thereby enabling the monopropellant fuel blend to be used in tactical systems deployed for use in cold environmental conditions. cl SUMMARY OF THE INVENTION
The combination of methanol (MeOH), hydrazine, and hydrazine nitrate yields a monopropellant blend having a lower freezing point and a higher specific impulse. This monopropellant fuel blend has a usefulness which extends to cold environments while at the same time it provides a higher specific impulse system. The addition of methanol, (MeOH) while miscible with hydrazine, acts to further reduce the freezing point of the monopropellant fuel blend. Since methanol contributes oxidizer function proportional to the oxygen content, a monopropellant fuel blend consisting of hydrazine, hydrazine nitrate, and methanol offers a variable specific impulse which is achieved by varying the amount of the hydrazine, hydrazine nitrate, and methanol. A suitable amount of MeOH to lower the freezing point temperature will achieve the desired physical properties. A suitable amount of MeOH to achieve the desired physical properties, the desired specific impulse, and the desired, lowered freezing point are considered in combination to arrive at the optimum values of each of the components of the fuel blend. Methyl alcohol has a carbon content of 37.48%, a hydrogen content of 12.58% and an oxygen content of 49.37%. Thus, the oxygen contribution of MeOH and the lowering of the freezing point of the composition are dual contributions of the additive MeOH, but the additional contributing attributes of the elements carbon, and hydrogen render MeOH a superior additive for use with hydrazine and hydrazine nitrate as compared with the additive water. The combination of 68 weight percent hydrazine, 20 weight percent hydrazine nitrate, and 12 weight percent MeOH provides a monopropellant fuel blend with a freezing point of -29.degree. F. A like amount of freezing point depression is achieved with the prior art additive water in an amount of 12 percent in combination with 20 percent hydrazine nitrate and 68 percent hydrazine, but the substitution of alcohol for water increases the energy available without adversely effecting the physical properties of the blend, i.e., viscosity values, and without adversely effecting the performance values, e.g. the small motor test data, and the differential thermal analysis (DTA) values.